The present invention generally relates to a method of making an inorganic bonding type rapidly solidified magnet by an energizing, sintering method, and a magnet rotor therefrom.
A rare earth--iron system rapidly solidified magnet is reported in, for example, a published paper, "Rare earth--Iron--Boron Materials; A new Era in Permanent Magnets" Ann. Rev. Sci., Vol. 16, p. 467-485 (1986) by J. F. Herbest and others.
According to the above described papers, an alloy molten bath including a rare earth element R, a representative transition metal element Fe/Co and B at a ratio near 2:14:1 provides a continuous splat from melt spinning, which is a rapidly solidified magnet thin piece having a microscopic structure with an R.sub.2 Tm.sub.14 B phase (where R is Nd/Pr, Tm is Fe/Co) of 20 through 400 nm at approximately 580.degree. C. in crystallizing temperature, dispersed in an amorphous Fe phase. The rapidly solidified magnet thin piece is magnetically isotropic. The representative magnet characteristic is a specific coercive force where Hcj&lt;8 kOe and a residual magnetization 4.pi.Ir.apprxeq.8 kG. But the material shape to be obtained from continuous splat quenching is restricted to a thin belt, thin piece or the like. Accordingly, to provide a magnet which can be employed as an electric motor, the operation for converting the material form, namely, fixing the powder of the rapidly solidified magnet thin piece by some method is necessary.
Although the fundamental fixing method in powder metallurgy is pressureless sintering, it is necessary to retain Hcj in accordance with the microscopic construction, so that the pressureless sintering of the rapidly solidified magnetic powder is hard to effect. As disclosed in, for example, U.S. Pat. No. 4,689,163 and U.S. Pat. No. 4,981,635, a resin bonded type rapidly solidified magnet for fixing the melt spun powder with resin is used to form a magnet for an electric motor. Hcj which can fix, under the crystallizing temperature or lower, the melt spun powder, remains basically invariable. But the resin bonding type rapidly solidified magnet is difficult to make with a density of 6g/cm.sup.3 or more and is restricted to 4.pi.Ir.ltoreq.6.2 kG.
There is a fixing method for rapidly filling a solidified magnetic powder into a mold so as to effect compression at the crystallizing temperature or greater, for accompanying plastic deformation. According to such method, the rapidly solidified magnet is made higher in density, i.e. near to the true density of the rapidly solidified magnet thin piece, while the level of the Hcj is retained to some extent. The rapidly solidified magnet is 4.pi.Ir.ltoreq.8.4 kG. If the rapidly solidified magnet is further deformed in plasticity, the magnetic anisotropy is caused according to the extent of distortion and becomes 4.pi.Ir.gtoreq.8.4 kG or more. Generally, a high frequency inductive heating operation of a mold is effected as a heating system to indirectly heat the melt spun powder in the mold. This method is not suitable for production on an industrial scale of a rapidly solidified magnet, because it takes more time to heat and cool the mold.
The rapidly solidified magnet with a rapidly solidified magnet powder having been energized and sintered in the mold, makes it possible to provide a near net shape with quick heat and a magnet of 4.pi.Ir.ltoreq.8.4 kG having a higher mechanical strength, with advantage, in an industrial scale operation.
In order to effect an energizing, sintering operation, the compression pressure of .ltoreq.200 kg/cm.sup.2 is applied to the melt spun powder in the mold, a current of 200-500 A/cm.sup.2 is fed to directly heat the rapidly solidified magnetic powder to deform it in plasticity at the crystallizing temperature or more, for making it minute. When an energizing operation is effected on a rapidly solidified magnetic powder (ring shaped) in the mold type formed into a ring shape, each portion in the peripheral direction of the ring shape may cause local current concentration without becoming equal in current density. Although the local current concentration difference has an effect on the surface condition of each thin piece construction of the melt spun powder, it is difficult to uniformly fill the melt spun powder into the mold formed in a ring shape.
When the local current concentration is caused in such ring-shaped peripheral direction as described hereinabove, the difference in the temperature rise of the melt spun powder is caused in accordance with the concentration. If the current concentration is slight, the temperature difference thereof is expanded together with the passage of the energizing time. The difference becomes largest at the highest temperature reached immediately after the energization current interruption. The temperature difference acts, in other words, to impart a difference in the roughness in the R.sub.2 Tm.sub.14 B phase. It means Hcj if the difference in 4.pi.Ir is slight in terms of the magnetic characteristics of the peripheral direction of the ring shaped magnet made minute, and change in the temperature factor of Hcj. When the extent of the current concentration is great, the mold itself breaks due to the temperature difference, so that a rapidly solidified magnet cannot be manufactured.
The Hcj level of the rotor magnet of the electric motor has an important influence on the reliability of the electric motor, such as demagnetizing proof force with respect to the reverse magnetic field by the excitation of the stator winding, and the temperature factor of the Hcj does demagnetizing proof force with respect to the temperature rise to be accompanied by the operation. Accordingly, the Hcj and the temperature factor of the Hcj must not change in the ring shaped peripheral direction. Further, when a brushless electric motor is driven in PWM, the high frequency magnetic field from the stator side corresponding to the carrier frequency is interlinked with a rotor magnet so as to generate eddy currents corresponding to the electric resistance of the rotor magnet. The loss caused by the eddy current raises the magnetic temperature so as to lower the output of the electric motor and the efficiency. For higher output and higher efficiency of such an electric motor, an effective rotator magnet higher in residual magnetization and higher in electric resistance is required.
A resin bonding type rapidly solidified magnet is employed in a magnet rotor (for example, U.S. Pat. No. 4,689,163 and U.S. Pat. No. 4,981,635). The resin bonding type rapidly solidified magnet has 4.pi.Ir.ltoreq.6.2 kG and an electric resistance of 10.sup.-4 -10.sup.-1 .OMEGA.cm.
The energized, rapidly sintered solidified magnet has a 4.pi.Ir&gt;6.2 kG and electric resistance 10.sup.-4 .OMEGA.cm (close to 10.sup.-5 .OMEGA.cm).
Accordingly, a higher 4.pi.Ir is obtained as compared with the resin magnet, but the electric resistance is low. Also, when tens to hundreds of watts from of rotator magnets are provided, the compression direction distance generally becomes longer. As the rapidly solidified magnetic powder is low in electric resistance, the energizing, sintering operation becomes difficult to effect as the compression direction distance becomes longer.
A ferrite sintered magnet has 4.pi.Ir.ltoreq.4.2 kG, electric resistance &gt;10.sup.4 .OMEGA.cm, and is lower in 4.pi.Ir as compared with the resin bonding type rapidly solidified magnet. Also, a rare earth cobalt system sintered magnet (SmCo.sub.5) has 4.pi.Ir.gtoreq.8 kG, electric resistance 10.sup.-5 cm and is lower in electric resistivity as compared with the resin bonding type rapidly solidified magnet.
As described hereinabove, a magnet which is superior in matching property between the 4.pi.Ir and the electric resistance suitable for a rotor magnet of, for example, a PWM driving brushless electric motor has, under the existing circumstances, a ferrite sintering magnet, and more preferably, 4.pi.Ir higher than it, is a resin bonding rapidly solidified magnet having comparatively high electric resistance. Generally these magnets dispose a non-magnetic reinforcing member in a magnetic circuit gap when necessary from the viewpoint of reliability in practical use, given the size, safety and the mechanical strength and so on of the magnetic rotor.